Inhibitors are antibodies to factor VIII which occur in 15-25% of patients with severe hemophilia A. Although it is known that race, the genetic defect causing hemophilia, and other genetic factors are somehow involved in inhibitor development, it is not currently possible to determine an individual's risk for developing an inhibitor. The purpose of this study is to use recent experimental findings about how infusion of plasma proteins such as factor VIII lead to antibody formation to develop a model for assessing individual risk for inhibitor development.